1. Field of the Invention
This invention relates to a water reducible epoxy resin curing agent and a method for preparing the curing agent and more particularly to water reducible amine functional epoxy curing agents suitable for crosslinking epoxy resins in water borne systems.
2. Description of the Prior Art
Epoxy resins are used as protective coatings for a variety of substrates which are exposed to harsh environmental or use conditions. The epoxy resin coating compositions have been applied as solutions in organic solvents. In view of the air pollution caused by the use of organic solvents and the increasing scarcity and cost of these solvents, there is a continuing search for new and improved epoxy coating compositions which can be reduced or dispersed in water.
In the commercial use of epoxy resin based coating compositions, it is desirable that these coating compositions cure or harden rapidly at room temperature. Water borne epoxy resin coating compositions known to the art generally require elevated temperatures of about 300.degree. to 500.degree. F. to effect hardening. For example U.S. Pat. No. 4,332,709 to Kooijmans discloses water soluble binders for can lacquers prepared by reacting an epoxy resin, specifically a polyglycidyl ether of polyhydric phenol with an aliphatic amino carboxylic acid such as glycine, alamine and glutamic acid, wherein the acid groups have been neutralized with a base such as an alkali metal hydroxide, tertiary amine or quaternary ammonium hydroxide. The coating composition comprising the epoxy resin and cross-linking agent is hardened by heating at 175.degree.-210.degree. C. (350.degree.-400.degree. F.) for 2-10 minutes.
U.S. Pat. No. 3,962,165 to Bosso et al discloses water dispersible electrodepositable quaternary ammonium salt containing resins based on polyglycidyl ethers of polyphenols. Once electrodeposited, the coatings are baked at 250.degree. to 500.degree. F. for 1 to 30 minutes to harden.
U.S. Pat. No. 4,246,087 to Tsou et al discloses a method for electrodepositing a film forming resin which is the reaction product of an oxirane containing material with a tertiary amine salt and optionally a fatty acid moiety. When combined with a melamine or a blocked isocyanate, the composition is electrodeposited and cured at elevated temperature.
U.S. Pat. No. 4,486,280 to Hosoi et al discloses a self-curing water dispersible epoxy resin composition produced by electron beam irradiation of a mixture of an epoxy resin and a quaternary alkyl ammonium salt of an .alpha.,.beta.-unsaturated carboxylic acid or a neutralized product of an .alpha.,.beta. unsaturated carboxylic acid and a tertiary amine. Aqueous dispersions of the coating composition are cured at 150.degree. C. (300.degree. F.) for 10 minutes.
U.S. Pat. No. 4,094,844 to Allen discloses hydrolytically stable water-soluble epoxy coating compositions curable with amino (melamine) resin curing agents prepared by (1) condensing an excess of a lower molecular weight epoxy resin with an amino-substituted benzoic acid, (2) reacting the terminal epoxide groups of the resulting condensate with a secondary amine and (3) solubilizing the product by reacting the carboxyl groups with a tertiary amine. The resultant epoxy coating composition is cured by heating for 2 to 10 minutes at 350.degree. to 400.degree. F.
U.S. Pat. No. 4,098,735 to Tobias discloses a water borne epoxy ester coating composition which is the adduct of a polyfunctional 1,2-epoxy resin and a monocarboxylic acid with a saturated dibasic acid and trimellitic anhydride, wherein free carboxyl groups are neutralized with ammonia or amine. The coating is cured by baking at 350.degree. to 450.degree. F. for 2 to 10 minutes.
The prior art also teaches water borne epoxy resin based coatings which are curable at room temperature. For example, U.S. Pat. No. 4,246,148 to Shimp et al discloses a room temperature curable, two component aqueous epoxy resin coating system in which there is dissolved a polyamine terminated epoxy resin endcapped with a monoepoxide and a micro-emulsified low molecular weight polyepoxide cross-linker.
U.S. Pat. No. 4,316,003 to Dante et al discloses epoxy curing agents useful in water borne, room temperature air dryable epoxy resin formulations prepared by first reacting an epoxy compound with less than a stoichiometric amount of a primary monoamine and then reacting the resulting condensate with a stoichiometric excess of a polyfunctional amine.
Although room temperature curing water borne epoxy resin coating systems have been in commercial use for some time, these systems have one or more of the following drawbacks: slow cure rates and dry times, poor early water resistance and short pot life.